The invention relates generally to methods and systems for coating a flexible substrate in order to achieve desired optical characteristics and more particularly to reducing the susceptibility of a titanium nitride film to cracking.
A wide range of single-layer coatings and multi-layer stacks are applied to light transmissive or reflective members in order to provide desired optical characteristics. For example, solar screening that increases the comfort level within the interior of a home, building or automobile can be obtained by providing the appropriate single-layer or multi-layer coating along the windows of the structure. A coating that provides solar screening is defined herein as one that has a relatively low transmissivity in both the visible range (400 to 700 nm) and the near infrared range (700 to 2100 nm). In comparison, wavelength selectivity is defined herein as referring to the relative transmissivities in the visible and near infrared portions of the solar spectrum.
It is known in the art to use metal nitride layers in window-energy control applications. Single layers of titanium nitride, zirconium nitride, and hafnium nitride have been specifically identified. As noted in U.S. Pat. No. 6,188,512 to Woodard et al., titanium nitride is particularly attractive, since titanium nitride films have desirable properties with respect to chemical stability and wavelength selectivity. The chemical stability of titanium nitride layers is well established in the art. Titanium nitride films are more transmissive in the visible portion (TVIS) than in the near infrared portion, so that the wavelength selectivity is greater than 1. The Woodard et al. patent describes an embodiment in which titanium nitride layers are formed on opposite sides of a particular substrate, significantly raising the ratio of the measure of transmission at the wavelength of 550 nm to transmission at the wavelength of 1500 nm.
Single-layer or multi-layer optical coatings that include titanium nitride may be formed directly on a glass substrate or may be formed on a polymeric substrate which is subsequently applied to glass. U.S. Pat. No. 6,114,043 to Joret describes a layer stack that is formed on a glass substrate intended to be used as a window for a building or an automobile. The layer stack may be a solar protection coating of three layers that are applied directly to glass. The first of the three layers is the titanium nitride layer. The second layer is formed atop the titanium nitride layer as a silicon nitride layer having controlled amounts of oxygen and carbon. Then, a layer of silicon oxycarbide is formed to provide the outermost layer of the layer stack. All three of the layers are formed using gas-phase pyrolysis techniques. The Joret patent states that one function of the center layer is to protect the titanium nitride layer when the glass substrate is subsequently cut and heat treated in order to bend or toughen the coated glass. Thus, if the coated glass is bent in order to provide the shape for the windshield of a particular make of automobile, the center layer will provide protection for the titanium nitride layer during the bending process.
Titanium nitride layers are particularly susceptible to damage when the layers are formed along the surface of a flexible substrate. In the above-identified patent to Woodard et al., the titanium nitride is deposited on polyethylene terephthalate (PET). A web of PET can be coated and then systematically cut in order to provide the desired shapes for coating the various windows of an automobile, for example. However, titanium nitride coatings are ceramic films in compressive stress. As a result, problems occur in coatings having a titanium nitride layer. In particular, when a coated polymeric substrate is flexed, the titanium nitride layer has a tendency to crack. The cracks often form a serpentine pattern, so that the cracking may be referred to as xe2x80x9cworm tracks.xe2x80x9d The thickness of the titanium nitride layer affects the susceptibility of the layer to cracking. A thin titanium nitride layer is less likely to crack. As one example, if a titanium nitride layer having a thickness of 7.4 nm is able to provide a TVIS of sixty percent, the layer is less prone to cracking as compared to a 20.4 nm layer that provides a TVIS of thirty-five percent.
What is needed is a means for reducing the susceptibility of a titanium nitride layer to stress cracking, while retaining the benefits of titanium nitride-based coatings, such as the benefits relating to wavelength selectivity and chemical resistance.
The structural stability of a titanium nitride-based optical coating on a generally transparent substrate is enhanced by providing a damage-retardation layer between the substrate and a layer of titanium nitride. The thickness of the titanium nitride layer is selected primarily for achieving desired optical characteristics, such as solar control, while the thickness of the damage-retardation layer is selected primarily for achieving desired mechanical characteristics. The damage-retardation layer is formed of a grey metal. The preferred grey metal is nickel chromium. The tendency of a titanium nitride layer to crack may be further reduced by exposing the substrate, such as a flexible polymeric substrate, to a plasma preglow and/or by using a slip agent on the side of the flexible polymeric substrate on which the grey metal and titanium nitride-based optical coating are formed.
In the solar control embodiment, the titanium nitride layer may define a single-layer optical coating having a visible light transmissivity (TVIS) within the range of ten percent to sixty percent. The damage-retardation layer and the titanium nitride layer are deposited upon a flexible polymeric substrate, such as a web of PET. Preferably, both layers are formed by sputter deposition. The grey metal material may be selected from at least one of the group consisting of Ni, Cr, Ti, W, Zr, Hf, Ta, Nb, Fe, V and Mo, as well as alloys, such as Monel, Inconel and stainless steel. Testing indicates that nickel chromium is a preferred material.
Less than all of the dynamics which lessen the tendency of titanium nitride to crack upon pressure of the substrate when the intermediate layer of grey metal is used are fully understood. Since the grey metal layer is more malleable than the titanium nitride layer, the grey metal layer may relieve some of the stress placed on the titanium nitride layer when the substrate is flexed. Moreover, because the grey metal will reduce light transmission, the titanium nitride layer may be reduced in thickness while the optical coating remains at the target level of TVIS. To obtain a target level of TVIS with a single material, a titanium nitride layer would have to be approximately three times thicker than a nickel chromium layer. Thus, the use of nickel chromium to form the damage-retardation layer allows the titanium nitride layer to be reduced to a thickness that is less prone to cracking. It is also believed that the nickel chromium layer serves as a primer layer and improves the adhesion of the titanium nitride layer to the polymeric substrate.
The plasma preglow that is used to improve structural stability of the titanium nitride-based coating may be performed in an environment in which the glow gas is one or a combination of argon, oxygen, or nitrogen. Plasma preglow treatments to polymeric substrates increase adhesion by generating desired functional groups or by simply increasing surface roughness. Either or both of these aspects may lessen the tendency of sputtered titanium nitride layers to crack and debond when the substrate is flexed.
It is known to apply slip agents to substrates to increase adhesion of inks, dyes or the like to the substrate. A slip agent is a chemical coating which provides an increased degree of surface roughness to PET and similar substrates. A slip agent is defined herein as an additive that is provided primarily to reduce the coefficient of friction of a substrate. Slip agents for polyester substrates are known in the art. As one example, glass spheres may be added to a polyester substrate or may coat the substrate to create a textured surface. In accordance with the present invention, the slip agent is employed to reduce the tendency of the sputter deposited titanium nitride layer to crack.
An advantage of the invention is that the structural stability of a titanium nitride-based optical coating for a transparent substrate is significantly enhanced. As applied to embodiments in which titanium nitride is used as a single-layer solar control coating of a flexible substrate, xe2x80x9cworm tracksxe2x80x9d are less likely to occur upon flexure of the substrate. Consequently, field failures of a solar control product are reduced.